# Difference between codes and representations

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 July 2, 2009, 11:59 Difference between codes and representations #1 Senior Member   Antonio Martins Join Date: Mar 2009 Location: Porto, Porto, Portugal Posts: 112 Rep Power: 8 Hi Foamers, I have a small question concerning relaxation. Is there any difference, when writting the code, between the following forms of relaxation, when solving the Navier stokes equation for laminar flow of newtonian fluids. Form 1 fvVector UEqn ( fvm::ddt(U) + fvm::div(phi,U) - fvm::laplacian(nu,U) ); UEqn.relax(); solve (UEqn == -fvc:: grad(p)); Form 2 fvVector UEqn() ( fvm::ddt(U) + fvm::div(phi,U) - fvm::laplacian(nu,U) ); UEqn().relax(); solve (UEqn() == -fvc:: grad(p)); Is there any significant difference between the two formulations. Thanks, Titio

 July 2, 2009, 14:10 #2 Assistant Moderator   Bernhard Gschaider Join Date: Mar 2009 Posts: 3,915 Rep Power: 40 No. The second UEqn is probably a autoPtr of a fvMatrix.

 July 3, 2009, 03:32 #3 Senior Member   Henrik Rusche Join Date: Mar 2009 Location: Braunschweig, Niedersachsen, Germany Posts: 275 Rep Power: 9 Hi Titio, Bernhard is absolutely right. Form 2 (and Form 1) are quoted incorrectly. Form 2 should read: tmp UEqn ( fvm::ddt(U) + fvm::div(phi,U) - fvm::laplacian(nu,U) ); (see simpleFoam) By using tmp (or alternatively autoPtr) one is able throw away the U-Matrix before solving the p-equation. This way, you can save some peak-memory in steady-state solvers. Regards, Henrik

July 3, 2009, 07:58
#4
Assistant Moderator

Bernhard Gschaider
Join Date: Mar 2009
Posts: 3,915
Rep Power: 40
Quote:
 Originally Posted by henrik Bernhard is absolutely right.
... and to short to be understood. I planned to add a second sentence "This is only an 'administrative' construct and doesn't change the physical meaning of the equation" and obviously forgot that.

But Henrik's answer is better anyway

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